There are basically two processes available for defatting and deoiling for example plant products: extraction and pressing. From an economic point of view pressing is only advantageous when the fat and oil content of the starting material is relatively high (&gt;25% by weight). However, even when this process is utilized optimally, a residual fat content of at least 4 to 5% by weight remains in the processsed residue. In contrast, extractive processes with normal organic solvents such as e.g. hexane or light petroleum are more suitable for starting products with a low fat content whereby the residual content in the residue can usually be reduced to under 1% by weight. A disadvantage of conventional solvent extraction is the fact that most solvents are not toxicologically harmless and that the extracted fats and oils as well as the extraction residues have to be substantially free of solvent before they can be used for foods or fodder. Comparatively high temperatures are necessary for this which can have a negative influence on the sensory properties of the products.
Separating has the greatest technical importance for defatting animal starting material such as e.g. fatty meat. After a thermal or mechanical disintegration of the educt, the difference in the density of the meat and fat components is usually utilized in this process in order to carry out a separation in separators or decanters. Using this process it is not possible for economic reasons to produce products from fatty meat that has a fat content of less than 10% since the losses of meat are too high. Therefore in order to produce meat products with a lower fat content it is necessary to use extractive processes which have the aforementioned disadvantages when organic solvents are used.
In order to circumvent this problem, extraction with compressed gases has been used in recent years in particular for the isolation of sensitive natural products whereby carbon dioxide (CO.sub.2) in particular has generally been accepted as the extraction medium in the technical field. The extraction of fats and oils with supercritical CO.sub.2 is only satisfactory in very high pressure ranges (&gt;500 bar) because of its low solubilizing power and this is technically complicated and consequently very cost-intensive and thus only comes into consideration for products for which there is a very high creation of value. This disadvantage can only be partially compensated by adding entraining agents to the CO.sub.2 and the additional expenditure for controlling the dosage of the entraining agent may be regarded as a further disadvantage.
Apart from CO.sub.2, compressed propane has also already been recommended as a solvent for the extraction of fats and oils. Thus according to DE-OS 28 43 920 crude vegetable fats and oils are refined with supercritical gases such as propane and CO.sub.2 while in other publications (U.S. Pat. No. 4,331,695=DE-OS 23 63 418, U.S. Pat. No. 3,939,281, DE-OS 22 55 567, DE-OS 22 55 566) an extraction pressure near to or above the critical pressure and subcritical extraction temperatures are recommended. The critical conditions for propane are .gtoreq.42 bar and .gtoreq.97.degree. C. Liquid propane is disclosed according to U.S. Pat. No. 2,560,935 and U.S. Pat. No. 2,682,551 as a solvent for oils and fats but no specific details are given with respect to the extraction pressure. Finally according to U.S. Pat. No. 2,254,245 a fat extraction is described at very low temperatures (&lt;0.degree. C.) whereas according to U.S. Pat. No. 1,802,533 a maximal extraction pressure of 7 bar is recommended. In addition critical state parameters were often selected for the separation of the extracted lipids in which a phase separation into a propane phase which is rich in oil and one which is poor in oil is utilized in this range of conditions in order to separate the oil or to fractionate it (cf. for example U.S. Pat. No. 2,660,590 or U.S. Pat. No. 2,548,434).
These known processes all have disadvantages: on the one hand extraction yields are obtained under critical and supercritical state conditions in which the quality of the extracted oil or fat is substantially reduced by high thermal stress. On the other hand although extraction with liquid propane is selective in pressure regions of &lt;10 bar, mass transfer is, however, limited during the extraction and therefore longer extraction periods and larger amounts of extracting agents are necessary in order to achieve the extraction goal for defatting and deoiling.